1
Optimization of the Pr doping in the (Bi
1.7
Pb
0.3
)(Sr
2

x
Pr
x
)CuO
6+
superconducting series
Y. C. Chu
1
,
H.

C. I. Kao
1
,
D. C. Ling
2
,
H.
S.
Sheu
3
and
T
.
S.
Chan
3
1
Department of Chemistry, Tamkang University,
Tamsui 251,
Taiwan
2
Department of Physics,
Tamkang U
niversity,
Tamsui 251,
Taiwan
3
National
Synchro
tron Radiation Research Center
, Hsinchu 300, Taiwan
*
virus777787
@
hotmail
.
com
Abstract
A series of Bi

2201
having
the nominal composition of (Bi
1.7
Pb
0.3
)(Sr
2

x
Pr
x
)CuO
6+δ
with
0
.050
x
0.50 was
prepared by the solid state reaction method.
All of them
have
the
orthorhombic
phase with a space group of
Amaa. Orthorhombicity, 2(a
b)/(a + b),
decreases with
increasing
the amount of Pr content.
The h
ighest T
c
(20.1 K) is
found in the
sample with
x = 0.4
0, which has an optimal hole concentration of 0.278(2) analyzed by an
iodometric titration
method
.
Hole concentrations with respect to the amount of Pr
substitution
.
Keywords:
Bi

2201, mixed valence, unit cell parameters,
superconductor
.
1.
Introduction
C
uprate
su
pe
rconductor
s have
interesting
and rich
characteristics of
the normal
state
conduction
behavior
. Basically,
all
s
uperconducting cuprates
have
active and non

active layer
intergrowth
in the
crystal
structure
[1].
T
he superconducting phase with a general formula
had been discovered in the Bi
2
Sr
2
Ca
n

1
Cu
n
O
6+
system [
2
–
4
]. They are the Bi

2201 (
n =
1),
the Bi

2212 (
n
= 2), and the Bi

2223 (
n
= 3), and their superconducting transition
temperatures are about 20, 80 and 1
00 K, respectively.
Bi
2
Sr
2
CuO
6+
(
BSCO
)
has a simple
structure with a single CuO
2
layer
.
The doping dependence on T
c
has been studied
extensively
since the hole concentration is easily changed by
partial
substitution
of Sr
by La
.
By
reduce the hole concent
ration
,
optimal T
c
of
Bi
2
(Sr
2

x
La
x
)
CuO
6+
δ
(BSLCO)
rolled up to 38
K
[
5
–
8
]
.
Bi

based superconductor posses
incommensurate modulation structure
due to the
extra
oxygen insertion along the unit cell b

axis [
9
–
11
].
which affects the superconductivity
and the
n
ormal

state transport properties
of the samples drastically
[
12
–
1
3
]
.
Pb
doping
leads
to the decrease of the modulation in (Bi
2

x
Pb
x
)Sr
2
CuO
6+
δ
(BPSCO) [14].
Co

doping of
Pb and
La
,
opti
miz
ation of the hole
concentration
while
reducing the
modulation
,
(Bi
2

x
Pb
x
)(Sr
2

y
La
y
)CuO
6+δ
(BPSLCO) has a maximum T
c
up to 40.3
K
[
1
5
].
In th
e 123 system
,
a series of 90 K superconductors with formulas as RBa
2
Cu
3
O
7

δ
were
observed, where R = rare earth, expect Ce, Pr and Tb [
16
]. All of Ce, Pr and Tb elements
have the 4+
valence state that is probably the reason that they can not be replaced into R site.
2
2.
Experimental
Bulk samples of
(Bi
1.7
Pb
0.3
)(Sr
2

x
Pr
x
)CuO
6+
were prepared
by
a
conventional solid

state
reaction method.
The
Pr
2
O
3
was preheated at
1000°C for
24
h and k
ept in a desiccator prior
to use. Stoichiometric amount
of Bi
2
O
3
, PbO, SrCO
3
, Pr
2
O
3
and CuO were weighted and
ground thoroughly
. Mixed powder was calcined at 805
°C
for
26
h in a box
furnace
with
five
intermittent grindings.
It was reground and pressed into
pellets,
sintered at
805
°C
for
24
h
and quench
ed
to
room temperature.
S
amples were checked for single phase formation by a
Bruker
MXP3
X

ray
D
iffractometer
equipped with a graphite
monochromator
.
Unit cell
parameters were determined by the Rietveld refin
ement method
.
GSAS (General Structure
Analysis System) developed by Larson and Dreele from Los Alamos National Laboratory
was employed for the structure analysis
[17
]
.
T
c
was found from the resistivity
versus
temperature curve measured by a standard
4

prob
e method. Oxygen stoichiometry and the
hole
concentration of the compound were determined by an
iodometric titration method [1
8
].
The
relative standard deviation obt
ained from titration
result
s for each sample is less than
1
%
.
3.
R
esult
s
and
di
scussion
Po
lycrystalline samples of
(Bi
1.7
Pb
0.3
)(Sr
2

x
Pr
x
)CuO
6+
were prepared with
0
.050
x
0.5
0
.
Fig. 1
shows the X

ray diffraction patterns in the 2
θ
range
between
0
°
and 50
°
for all
the compositions in
(Bi
1.7
Pb
0.3
)(Sr
2

x
Pr
x
)CuO
6+
series.
All of them are single

phase
compounds with orth
orhombic crystal system. Purity
of the samples were carefully examined
by XRD
patterns in the range of
3
0
° ≤
2
θ ≤
3
5
°
. Impurity
phases
Sr
CuO
2
+
and
rel
ated
compounds, were not found.
The space group of Amaa is used for the Rietveld analysis
for
the single phase samples of
(Bi
1.7
Pb
0.3
)(Sr
2

x
Pr
x
)CuO
6+
with 0.050
x
0.50. The R
wp
obtained from the Rietveld refinement o
n
this samples is between 8
10%.
Fig. 2 shows a
refinement result for x =
0.4
0
sample which is typical for all the (Bi
1.7
Pb
0.3
)(Sr
2

x
Pr
x
)CuO
6+
samples.
The
figures include the experimental, calculated, and difference XRD profiles.
Crosses are the experime
ntal data; solid lines are the calculated profile. All the possible
Bragg reflections are indicated with short vertical tics below the calculated profile. The
difference between the experimental and
calculated results is plotted below the Bragg
reflection
tics.
Unit cell axes as a function of x are plotted in Fig.
3
.
Both the
a

and
c

ax
is
d
e
crease with increasing x
,
while the
b

axis
in
creases with increasing x
,
indicating a
successfully substituting of Pr into the Sr site
.
The results of
Fig.
4
has been fo
und by
an
iodometric titration that the
oxygen
stoichiometry (
y)
and formal valence of Cu
(p)
in the (Bi
1.7
Pb
0.3
)(Sr
2

x
Pr
x
)CuO
6+
.
The
oxygen content was estimated by the average Cu valence under the assumption that Bi
3+
, Sr
2+
,
Pb
2+
, Pr
3+
and O
2

remained f
ixed valence. In the present stage, the valence of Cu indicates the
formal valence.
y
in
creases with
increasing x
from
6
.
205
(2)
for x = 0
.050
to 6.
36
9(1)
for x =
3
0.
50
.
The average hole co
ncentration (
p
) found in the
c
ompounds is in the range of 0.2
37(2)
≤
p
≤
0.3
6
0(3)
,
p
decrease
s
with increasing Pr content
.
Introducting Pr into the Sr site
, it
is a
hole filling effect and carrier
concentrati
on is varied
with
the Pr content
.
T
c
and hole concentration of the compound (p) in
the
BPSPCO
series relations
hip is
shown in
Fig.
5
.
T
he optimal T
c
of 20
K
with
p = 0.2
78
(2
)
is found in the sample with x =
0.40
.
A
cknow
ledgment
This work is financially supported by the National
Science Co
uncil of
Taiwan.
References
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ao
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4
Fig. 1.
XRD patterns
o
f
(Bi
1.7
Pb
0.3
)(Sr
2

x
Pr
x
)CuO
y
samples
with (0.050
x
0.50)
.
10
20
30
40
Intensity (a.u.)
2
(
o
)
Fig.
2. Rietveld refinement result of a
(Bi
1.7
Pb
0.3
)(Sr
2

x
Pr
x
)CuO
6+
with x = 0.
4
0. The
figures
include the experimental, calculated, and difference XRD profiles. Crosses are the
experimental data; solid lines are the calculated profile. All the possible Bragg reflections are
indicated with sho
rt vertical tics below the calculated profile. The difference between the
experimental and calculated results is plotted below the Bragg reflection tics.
Fig.
3
.
Unit

cell axes dependence with x
for
the
(Bi
1.7
Pb
0.3
)(Sr
2

x
Pr
x
)CuO
y
samples
.
0.00
0.15
0.30
0.45
5.320
5.355
5.390
24.43
24.50
24.57
Axis (A)
x
a
b
c
10
20
30
40
50
x=0.50
x=0.45
x=0.425
x=0.40
x=0.375
x=0.35
x=0.30
x=0.25
x=0.20
x=0.15
x=0.10
Intensity (a.u.)
x=0.05
2
(
o
)
5
0.00
0.15
0.30
0.45
6.20
6.25
6.30
6.35
0.20
0.25
0.30
0.35
0.40
p
y
x
F
ig.
4
.
Oxygen stoichiometry
(
y
)
and
hole concentration
(
p
)
dependence with x
for
(Bi
1.
9
Pb
0.
1
)(Sr
2

x
Pr
x
)CuO
y
samples
.
(
The lines drawn through
the dat
a
are
guides
to the eye
)
.
2.196
2.208
2.220
2.232
0
7
14
21
Tc
Relative intensity (Cu)
F
ig.
5
.
T
c
dependence with
hole concentration (p
) for
(Bi
1.
9
Pb
0.
1
)(Sr
2

x
Pr
x
)CuO
y
samples
.
(
The lines drawn through
the dat
a
are
guides
to the eye
)
.
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